1. Field of the Invention
The present invention relates to a temperature sensor, and particularly to a temperature sensor having a high accuracy in high-temperature regions.
2. Description of the Related Art
In recent years, catalysts for exhaust gas have been employed in an effort to cope with environmental hazards and the like presented by exhaust gas of automobiles. Such catalysts require accurate measurement of exhaust gas temperature in order to maximize their performance. Therefore, a temperature sensor which is highly accurate especially in high-temperature regions has been desired.
FIG. 1 shows an exemplary configuration for a temperature sensor. The temperature sensor includes a thermistor element 1 sealed in a heat-resistant cap 5. As shown in FIG. 2, Pt (platinum) pipes 2a and 2b are inserted into the thermistor element 1, the Pt pipes 2a and 2b being sintered with the thermistor 1 in an integral manner. Lead wires 4a and 4b of a dual-core pipe 3 are welded to the Pt pipes 2a and 2b. Thus, the lead wires 4a and 4b extend from the heat-resistant cap 5.
In order to obtain a highly accurate temperature sensor, a large thermistor constant B is desired, the thermistor constant B representing the resistance temperature characteristics of the thermistor element 1. The thermistor constant B is defined by Equation 1 shown below. A larger value of the constant B indicates a larger change in the resistance value of the thermistor in response to temperature changes. EQU B=[ln(R.sub.1)-ln(R.sub.2)]/(1/T.sub.1 -1/T.sub.2) (K) Eq. 1
where
T.sub.1 represents a temperature (K); PA1 T.sub.2 represents a temperature not equal to T.sub.1 (K); PA1 R.sub.1 represents a resistance value (.OMEGA.) of the thermistor at temperature T.sub.1 (K); and PA1 R.sub.2 represents a resistance value (.OMEGA.) of the thermistor at temperature T.sub.2 (K).
In order to increase the reliability of the measurement, it is preferable that the resistance value of the thermistor element has little change over time.
Conventionally, a thermistor element for a temperature sensor for high-temperature use where the maximum working temperature (i.e., the maximum temperature at which the temperature sensor is used) exceeds 700.degree. C. is typically composed of a corundum type thermistor material such as (Al, Cr, Fe).sub.2 O.sub.3. However, such conventional thermistor elements have a low constant B, e.g., 5000 K. in the temperature range between 300.degree. C. to 900.degree. C., indicative of inappropriateness for temperature measurement in such relatively high temperature regions. Also, it has been impossible to attain a high constant B value in a broad range of working temperatures although it has been possible to obtain a high constant B value at a specific working temperature.
Consequently, there has been desired a temperature sensor which attains a high constant B value in a broad range of relatively high temperatures and therefore has a high accuracy. In particular, a temperature sensor is desired in which the resistance value has little change over time during high-temperature use and therefore has high measurement reliability. Accordingly, there has been a strong demand for thermistor materials for thermistor elements to be used for such high-temperature sensors.